Exhaust gas discharged from an internal combustion engine of, for example, an automobile contains toxic components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Hitherto, three-way catalysts have been used for removing such toxic components for detoxifying the exhaust gas.
Such a three-way catalyst employs a noble metal such as Pt, Pd, or Rh serving as a catalytic active component; alumina, ceria, zirconia, a ceria-zirconia complex oxide having an oxygen-occluding ability, or the like serving as a carrier; and a catalyst support which is made of a ceramic or metallic material and which is in the form of a honeycomb, a plate, a pellet, or the like. Generally, when the excess air ratio λ is approximately 1, three-way catalysts employed in gasoline automobiles can maintain very high performance of removing CO, HC, and NO, whereas when the excess air ratio λ is higher than 1 (i.e., under a lean condition), the NOx removal performance drops considerably due to excessive oxygen. In order to improve the NOx removal performance when the excess air ratio λ is higher than 1 (i.e., under a lean condition), an NOx trapping catalyst is generally employed. However, it has been known that when high-sulfur-content gasoline is used, the catalyst is prone to undergo poisoning with S, resulting in a drop in NOx trapping performance.
There has been proposed an exhaust gas purification catalyst which can remove NOx at high efficiency while minimizing formation of N2O even under an exhaust gas having a low HC/NOx ratio. The proposed catalyst has a double-layer structure which includes a first catalyst layer containing, as a carrier, a phosphate salt (e.g., magnesium phosphate, silver phosphate, nickel phosphate, copper phosphate, iron phosphate, zinc phosphate, or tin phosphate) having excellent S-poisoning resistance and serving as an HC-reforming material; and a second catalyst layer containing Rh-supporting and WO3-modified alumina (or silica-alumina). In operation, partial oxidation of HC contained in the exhaust gas by the HC-reforming material is catalyzed at an air fuel ratio under a lean condition, to thereby form oxide-containing compounds (see Patent Document 1). There has also been proposed an NOx removal catalyst having a carrier formed of at least one compound selected from a metal carbonate, a metal sulfate, and a metal phosphate, and iridium serving as an active metal supported on the carrier (see Patent Document 2).